1. Field of the Invention
The present invention relates to a golf ball. More specifically, the present invention relates to a golf ball with a hollow metal core.
2. Description of the Related Art
The Rules of Golf, as set forth by the United States Golf Association (“USGA” and the Royal and Ancient Golf Club of Saint Andrews, have placed controls on the construction and performance of golf balls. The golf ball rules require that the golf ball have a diameter no less than 1.68 inches (42.67 mm), a weight no more than 1.620 ounces avoirdupois (45.93 g), spherical symmetry, an overall distance no greater than 296.8 yards (the limit is 280 yards, or 256 m, plus a six percent tolerance for the total distance of 296.8 yards), and an initial velocity no greater than 255.0 feet per second (the limit is 250 feet or 76.2 m, per second with a two percent maximum tolerance that allows for an initial velocity of 255 feet per second) measured on a USGA approved apparatus.
The initial velocity test is comprised of a large 200 pound wheel that rotates around a central axis at a rate of 143.8 feet per second (striker tangential velocity) and strikes a stationary golf ball resting on a tee. The wheel has a flat plate that protrudes during its final revolution prior to impact with the golf ball. The ball's velocity is then measured via light gates as it travels approximately six feet through an enclosed tunnel. Balls are kept in an incubator at a constant temperature of 23 degrees Celsius for at least three hours before they are tested for initial velocity performance. To test for initial velocity, balls are placed on a tee and hit with the metal striker described above. Twenty-four balls of a particular type make up one test. Each ball is hit with the spinning wheel a total of four times. The highest and lowest recorded velocities are eliminated and the remaining two velocities are averaged to determine the ball speed for that specific ball. The individual speeds of the 24 balls in the group are then averaged, and that is considered the mean initial velocity (IV) of the group for the test.
For USGA conformance purposes, a ball with a mean initial velocity of less than 255.0 feet per second is considered conforming to the USGA Rule of Golf and can be played in sanctioned events. For reference to USGA Wheel Test see USGA web-site at www.usga.com, or reference U.S. Pat. No. 5,682,230 for further information.
Generally speaking, the USGA IV test is designed to be a consistent measurement tool capable of regulating the speed (and ultimately distance) of golf balls. It is commonly known in the industry that golf ball manufacturers perform a simpler test on prototype golf balls and then attempt to correlate the results to the USGA Wheel Test. One type of correlation test is the Coefficient of Restitution (“COR” test, which consists of firing a golf ball from a cannon into a fixed plate and taking the ratio of outgoing velocity to incoming velocity.
The Coefficient of Restitution is the ratio of the velocity of separation (Vout1−Vout2) to the velocity of approach (Vin1−Vin2), where COR=(Vout1−Vout2)/(Vin1−Vin2). The value of COR will depend on the shape and material properties of the colliding bodies. In elastic impact, the COR is unity and there is no energy loss. A COR of zero indicates perfectly inelastic or plastic impact, where there is no separation of the bodies after collision and the energy loss is a maximum. In oblique impact, the COR applies only to those components of velocity along the line of impact or normal to the plane of impact. The coefficient of restitution between two materials can be measured by making one body many times larger than the other so that m2 (mass of larger body) is infinitely large in comparison to m1 (mass of the smaller body). The velocity of m2 is unchanged for all practical purposes during impact andCOR=VOut/Vin 
Most recently, DuPont Chemicals has developed a new material sold under the HPF tradename, which is composed of copolymers or terpolymers with a fatty acid salt. Aspects of the HPF™ material are discussed in U.S. Pat. No. 6,653,382. The HPF™ material increases the coefficient of restitution of the golf ball.
It would be preferred to use the HPF material for a one-piece golf ball. However, the specific gravity of HPF is such that the mass of the golf ball would not be sufficient. Additionally, if tungsten is added to a mixture of HPF material, the material becomes black in color and is difficult to paint.